Periodicity-Enhanced Attenuating Layers and Structures

Periodicity-enhanced (meta) materials and surfaces are artificial structures that possess properties not found in naturally-occurring materials and surfaces. The periodicity stems from the regular spacing of inclusions in a host matrix or roughness on a surface. Inclusions range from solid cylinders in air such as encountered in 'sonic crystals' to a grid framework in a poroelastic material such as an air-filled foam used for sound absorption. Roughness elements can be of various shapes and profiles ranging from identical rectangular grooves to arrays with fractal profiles. Without further modification, periodicity-enhanced materials stop the passage of some incident wavelengths (or frequencies) and enhance the transmission of others. By modifying the roughness of a surface, the interference between waves travelling directly from a source to a receiver above the surface and waves reflected from the surface can be controlled.
The proposal is concerned with ways of extending the frequency range over which the periodicity-enhanced materials and surfaces reduce the transmission of sound and vibration. The methods to be investigated include use of locally resonant inclusions or roughness elements, use of multiple resonances, exploitiation of interactions and overlaps between resonances periodicity-related transmission loss and spatial variation of periodicity and other characteristics thereby producing graded systems and roughness profiles. The work will provide a basis for the design of more efficient sound and vibration absorbing devices that are lightweight yet offer high transmission loss and vibration damping properties. The resulting surface designs will include alternatives to conventional noise barriers, while allowing access and preserving line of sight, and cost-effective methods for protecting buildings against ground-borne vibrations.

Metamaterials are materials with properties not found in natural or other manmade materials. So far most acoustical applications of metamaterials relate to super lensing, negative refraction and cloaking. One way in which these exceptional properties are obtained is through periodic inclusions. The primary focus of this proposal is on developing the ideas of periodicity-enhancement and graded refractive indices to design materials and structures that are useful for noise and vibration control. External costs of noise in the EU amount to at least 0.35% of its GDP. Excessive noise is considered to be among the most important environment and health problems ranking just behind the impact of air quality. First conservative and partial estimates show that at least 1.600.000 disability adjusted life years lost every year in the EU may be attributed to road traffic noise.The proposed research will contribute to the engineering challenge of designing lightweight materials and structures for reducing sound and vibration particularly at low frequencies. By concentrating on the development of practical and robust designs the proposed work will broaden the scope of the planned EPSRC expansion of activities in metamaterials.
Although the use of periodicity-enhanced metamaterials for noise and vibration control offers nearly endless possibilities, it is unlikely that all of them will become reality. One aim of this project is to evaluate and compare the feasibility of different approaches to acoustical metamaterial design and thereby prepare the ground for their commercial exploitation. Two fundamental questions we aim to answer are (i) do metamaterials for noise and vibration control offer a significant improvement in performance compared with traditional acoustic treatments to justify the efforts and extra costs involved in their design and manufacturing? (ii) can they be designed to be significantly robust, for example if intended for outdoor use, so that the properties of their basic elements (meta atoms) will not degrade over time. The significantly positive answers to these questions that should stem from the proposed research will benefit the economy and society in general since the resulting metamaterial structures could replace or enhance virtually any type of the existing noise and vibration treatments and lead to exploitable outcomes in three sectors of the economy including Environmental Protection, Construction and Transport. While traditional noise barriers are considered the most effective and cheap way of shielding residential areas from traffic noise, for example along motorways, metamaterial screens could provide a thinner alternative that is transparent to air flow and hence less susceptible to the air turbulence effects around the edges. The surface and buried periodicity-enhanced structures to be developed in this proposal will provide more effective protection from the traffic induced ground-borne vibrations than the digging of a trench or the installation of a massive foundation. In this respect the work is relevant also to the recent EPSRC call on ground engineering.
Metamaterial screens could be used, in schools and airports for example, as light and foldable partitions for reducing the impact of noisy environments. Metamaterial absorbers could be used in hospitals and food preparation areas where health and safety regulations make the use of traditional porous absorbers problematic. Graded index metamaterial structures could replace traditional motor enclosures and the use of lightweight periodicity enhanced absorbers could contribute into the reduction of vehicle weight.
Also the design of new materials for the control of noise and blast is an 'area of high interest to the MOD' (see DSTL Letter of Support).